The French term for sticking a paper fish on someone’s back on April Fools’ is “Poisson d’Avril.”
Literally translated as “April fish,” poisson d’avril is the French name for the April 1 prank tradition as well as the exclamation used when a joke is exposed. The classic prank is to fasten a paper fish to someone’s back secretly, allowing the person to walk around unaware of it.
The custom is especially associated with France, but it also appears in French-speaking areas of Belgium, Switzerland, and Canada. French bakeries, chocolatiers, and confectioners may mark the day by displaying fish-shaped sweets, extending the joke into a seasonal food tradition.
The origin of the fish is not settled. Proposed explanations connect it with Lent and the prominence of fish during the fasting season, fishing-season customs, the zodiac sign Pisces, and older spring festivals. The expression itself is documented in French literature before the popular theory that it began after France’s 16th-century calendar reform.